Let's enable AppArmor by default (why not?)

[intrigeri]

Hi!

tl;dr: I hereby propose we enable AppArmor by default in testing/sid,
and decide one year later if we want to keep it this way in the
Buster release.

My goals when initiating this discussion are:

- Get a rough idea of what amount of effort the Debian project is
happy (and able) to invest into such proactive security measures.

- Learn about any relevant social & technical concerns I am not
aware of.

I don't expect we'll make a decision based on this very proposal:
I expect the proposal will need to be refined, or abandoned, to take
into account what we will learn during this preliminary discussion.

Why do we need AppArmor?
========================

AppArmor is a Mandatory Access Control framework implemented as
a Linux Security Module (LSM), user space utilities, and a quite
simple language to define policy.

AppArmor confines programs according to a set of rules that specify
what operations a given program can access, e.g. it can prevent
exploited server software from accessing data it does not own and
executing arbitrary code. This proactive approach helps protect the
system against some known and unknown vulnerabilities.

Various actors are actively exploiting software. Random users are
victimized every day, and specific populations are specifically
targeted, e.g. government opponents, human rights defenders, system
administrators, software developers and distributors, as revealed by
the Snowden leaks.

Every month we learn about many new attack vectors made possible by
programming errors. We fix them after the fact, which is great but
a bit too late: users may already have been exploited. Most operating
systems have adopted proactive approaches to mitigate the impact of
such problems.

In Debian, great efforts are in progress: hardening binaries makes it
harder to write successful exploits, and making our packages build
reproducibly will make it harder to introduce vulnerabilities at the
binary level.

Still, Debian is far from being best in class on this front: we have
no widespread mechanism for sandboxing desktop applications. We can
surely do better. The great news is that there is one low-hanging
fruit waiting to be picked, and it's what this proposal is about :)

A proposal
==========

1. Enable AppArmor by default in testing/sid as soon as feasible in
the Buster cycle.

I can think of several possible ways to do it but for now I'd
rather focus on the "do we want to do it at all" conversation.

If curious some options are listed on the wiki:
https://wiki.debian.org/AppArmor/Progress#Enabling_AppArmor_by_default.3F
Feel free to discuss them on <pkg-appa...@lists.alioth.debian.org>.

And anyway, you can already opt-in for AppArmor on your system today:
https://wiki.debian.org/AppArmor/HowToUse :)


2. During a year, watch out for AppArmor related issues and address
them in a prompt manner.

Note that the best way to address them quickly enough is sometimes
to simply disable the problematic AppArmor profile: it's cheap,
doesn't require advanced AppArmor skills, and IMO a smaller
AppArmor policy enabled by default is more useful than a broader
but less robust one that only a couple thousand users benefit from.

3. A year after AppArmor was enabled by default: evaluate how it went
and decide if Buster should be shipped with AppArmor enabled by
default or not.

I commit to do an analysis using BTS data to help make this
decision. If we need formal success criteria and a clearly defined
team who'll make the call, I'm happy to think about it. But here
again I'd rather focus on the general idea than on implementation
details at this point.

Questions and Answers
=====================

Table of contents:

- What's the benefit, exactly?
- What do other distributions do?
- What's the history of AppArmor in Debian?
- How popular is AppArmor in Debian?
- What's the cost for Debian users?
- What's the cost for package maintainers?
- Is the Debian AppArmor team strong enough to support this change?
- Why AppArmor and not SELinux?
- Why AppArmor and not sandboxing based on XYZ?
- Will this prevent users from using another Linux Security Module?
- What does upstream look like?
- How much will we depend on Canonical's business priorities?
- No thanks: I've tried AppArmor and it broke stuff too often
- Doesn't AppArmor need out-of-tree kernel patches?
- How can I help?
- Credits

What's the benefit, exactly?
----------------------------

Before we even bother looking at the cost of enabling AppArmor by
default, let's look closer at the expected benefit. In other words:
what kind of attacks does AppArmor really mitigate or prevent in the
real world?

tl;dr: big benefit for server software, and for desktop applications
limited to less sophisticated, non-targeted attacks (but it'll get
better).

AppArmor is well suited to protect against remote exploitation of
security issues in server software and non-GUI programs often run with
elevated privileges (think of dhclient, ping, tcpdump). I'm sure one
could identify a few serious issues that would have been mitigated or
prevented by our current AppArmor policy, by looking at a list of
DSA/CVE. Also, one gets interesting security properties when software
is tuned for AppArmor: e.g. a given libvirt/QEMU virtual machine can
only access its assigned storage volumes, and not other VMs'; this is
useful against QEMU security issues that allow guests to escape the
virtualization layer.

On the desktop, to be honest things look pretty bad *currently*:
AppArmor is not enough, and we need new concepts and technologies to
fix serious limitations on the desktop sandboxing front.
Thankfully this is being actively worked on and the future of desktop
sandboxing on GNU/Linux looks bright and shiny! Some of the future
options rely on AppArmor, some others don't. See the "Why AppArmor and
not sandboxing based on XYZ?" section below.

Still, confining desktop apps with AppArmor has benefit against
scripted or non-targeted attacks: it will mitigate some attacks and
others will get through. So while it's probably not worth starting to
write lots of new policy for GUI applications now, I think that
leveraging the existing one will already serve our users.

What do other distributions do?
-------------------------------

AppArmor has been enabled by default in several other GNU/Linux
distributions and Debian derivatives for a while:

* in openSUSE + SLES since 2006
* in Ubuntu since 2008, with a growing policy:
https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/AppArmorProfiles
* in Tails, since 2014 for a few important services (CUPS, Tor) and
a few desktop applications (e.g. Totem, Evince, Pidgin, Tor
Browser, Thunderbird)
* in a few other Debian derivatives (Whonix, Subgraph OS) for at
least a couple years.

What's the history of AppArmor in Debian?
-----------------------------------------

AppArmor has been available (opt-in) in Debian since 2011. In 2014
a Debian AppArmor packaging team was created, that has been taking
care of the AppArmor packages and policy since then.

In the last 3 years the AppArmor policy shipped in Debian was extended
substantially and its coverage is now on par with Ubuntu's. It's still
rather small due to the strategy we chose: we wanted to avoid
traumatizing early adopters and to avoid creating a culture of
"AppArmor always breaks stuff, let's get used to disabling it".
So like Ubuntu and openSUSE, we're shipping a rather small and mature
AppArmor policy. I believe this strategy has been successful so far,
and even some non-trivial pieces of software like Thunderbird now ship
an AppArmor policy; but of course it has one drawback: most software,
including web browsers, is not confined with AppArmor whatsoever.
Surely with more people contributing to our AppArmor policy we could
have it cover other important pieces of software; time will tell.

A number of maintainers accepted shipping AppArmor policy in their own
package. If you're one of them, please consider providing feedback
about how it went for you.

How popular is AppArmor in Debian?
----------------------------------

tl;dr: AppArmor has steadily become more and more popular in Debian in
the last few years. I think the user base has reached a critical mass
that proves it works OK.

Here's what popcon says ("Vote" count) for the apparmor binary
package, that's needed to use AppArmor:

* 2015-01: ~400
* 2016-01: ~700 (+75% in a year)
* 2017-01: ~1300 (+85% in a year)
* today: 1870 (+44% in 7 months)

But we have no way to tell whether a user who has AppArmor packages
installed actually enabled the AppArmor LSM, so the data for
apparmor-profiles-extra might be more useful here: I expect that only
users who really want to use AppArmor with an extended policy would
bother installing it. This one has 435 registered installations
("Vote" has always been 0 for some reason that I did not investigate);
it was introduced in October 2014, and since then its popcon stats
have been steadily increasing.

What's the cost for Debian users?
---------------------------------

AppArmor unavoidably breaks functionality from time to time: e.g.
new versions of software we package (or of their dependencies)
regularly start needing access to new file locations.

And then users see broken applications from time to time, after
upgrading their testing/sid system. This is to be taken seriously, but
not a big concern IMO:

- Apparently Ubuntu users have been coping with AppArmor enforced
by default for 9 years. I see no reason why Debian users would not.

- I've counted 14 bugs bugs reported in the Debian BTS during the
Stretch development cycle against our supported AppArmor policy.
Among those, 11 were closed (106 days after being reported on
average); all the important ones were closed within 2 months;
larger delays were due to users developing fixes and/or upstream
taking some time to review merge requests. About the 3 bugs still
open: one is waiting for input from other package maintainers since
2 years, another one had a patch waiting to be applied, and the
last one needs to be fixed in libvirt upstream.

- Serious breakage is less likely to happen once AppArmor is enabled
by default, as there are greater chances that the maintainer would
have noticed it before uploading.

- Workarounds are regularly suggested to the bug reporter on the BTS,
and in many cases the bug reporter documents in the bug report the
workaround they have *already* applied.

Implementing a suggested workarounds requires being able to edit
a text file and running one command as root, which should be doable
by the vast majority of testing/sid users.

What's the cost for package maintainers?
----------------------------------------

For most of them: none at all. As said earlier, our AppArmor policy
does not cover that much software yet.

But maintainers of software confined by AppArmor will have to deal
with a new kind of bug reports, whose number is likely to grow
significantly once AppArmor is enabled by default. This means they
have to:

1. identify if a bug report can possibly be related to AppArmor;

2. either learn how to debug AppArmor issues themselves, or ask the
pkg-apparmor team for help (thanks to usertags:
https://wiki.debian.org/AppArmor/Reportbug#Usertags).

I expect that initially pkg-apparmor will need to provide help in many
cases, but over time the affected maintainers will slowly learn just
enough about AppArmor to handle at least the simplest cases
themselves, just like it happened in Ubuntu years ago.

Is the Debian AppArmor team strong enough to support this change?
-----------------------------------------------------------------

This is a valid concern, as I have been doing the greatest part of the
work on this team.

So far I've found my AppArmor-related workload totally sustainable:
it took me just a few hours here and there, and I would be doing this
work for Tails anyway, so better do it directly in Debian. Still,
primarily relying on one single person is concerning.

Thankfully, a number of other people have been contributing in various
ways. A few Debian users and contributors got used to reporting bugs
and contribute improvements to our AppArmor policy upstream.
Another team member uploaded src:apparmor once. Ulrike Uhlig wrote
documentation about AppArmor for Debian users and
contributors during an Outreachy project whose outcome was posted to
debian-devel-announce in March, 2015.

Also, just like any such distro-wide change, I expect the amount of
work required to support the broader project:

- will be large initially; I'm confident that the current state of
our team is good enough to support the project during the first
stage of the transition;

- will only decrease over time, as Debian people get used to it and
learn the small bits they need to know about the new technology,
and eventually the cases when our AppArmor team has to give a hand
will become rare;

- will be done by AppArmor people from other distributions as well:
a few of them actively participate on the pkg-apparmor mailing list
and help on issues reported in the Debian BTS.

So I think it's totally reasonable to give it a try.

Why AppArmor and not SELinux?
-----------------------------

SELinux is another LSM that tackles similar problems.

Disclaimer: I've picked AppArmor years ago and didn't look much at
SELinux recently, so some of what follows may be totally wrong or
outdated. Sorry! Debian SELinux people, if you don't mind please help
me get the basic facts right :)

Pros:

* Allows mediating more kernel objects / interfaces than AppArmor, so
policy can be made stricter and safer given sufficient expertise
and available time for maintenance.

* Enabled by default in RHEL so in theory a great number of sysadmins
are at ease with it (see below why reality may not match this).

* A quick look at popcon suggests that SELinux might be more popular
in Debian than AppArmor, but I'm not sure I am interpreting the
numbers right (and I suspect that just like AppArmor, the popcon
won't tell us if users who have installed the relevant support
packages actually run their system with the corresponding LSM
enabled & enforced).

Cons:

* Writing, maintaining, auditing and debugging SELinux policy
requires grasping a complex conceptual model; I am told this is not
as easy as doing the same with AppArmor.

* As far as I could understand when chatting with sysadmins of Red
Hat systems, this has resulted in a culture where many users got
used to disable SELinux entirely on their systems, instead of
trying to fix the buggy policy. I've seen the opposite happen with
AppArmor, which is good: for example, pretty often bug reporters to
the Debian BTS document themselves how they could workaround the
problem locally *without* turning AppArmor off. Looking at open
bugs in the BTS against src:refpolicy, this seems to happen very
rarely for SELinux, so I wonder if it would be realistic to ship
Debian with SELinux enforced by default and have our community
support it.

* https://wiki.debian.org/SELinux/Issues says "Graphical/Desktop
installs of Debian are not heavily tested with selinux, so you
might run into quite some issues".

* I'm not aware of any Debian derivative shipping with SELinux
enabled by default. If that's correct, then it means that we would
have to deal with quite some policy compatibility issues ourselves.

To me, the complexity of SELinux is a deal breaker: it seems that we
would need to get lots more expertise and energy to enforce SELinux by
default than doing the same with AppArmor.

Now, if for some reason the project prefers to ship with SELinux
enforced instead of AppArmor, fine by me: I would strongly prefer this
option to nothing at all.

Why AppArmor and not sandboxing based on XYZ?
---------------------------------------------

(Replace "XYZ" with Flatpak, Ubuntu's Snappy, Subgraph OS' oz,
Firejail, Subuser, or you preferred other promising option.)

We need both!

AppArmor covers server software well, and on the desktop it currently
protects against not-too-sophisticated, non-targeted attacks.

In a nutshell, the GNU/Linux desktop really wasn't designed for
applications to be siloed. To fix that we need new concepts and
technologies, such as Wayland, portals, and fine-grained D-Bus
mediation. Next generation desktop sandboxing technologies will fix
this and improve UX at the same time, and it will be amazing. But they
are not ready for prime-time yet. A Debian user cannot benefit from
them *today* much; this might change in time for Buster, but really
we're comparing a well-established, polished solution with a bunch of
other ones whose integration with Debian is being brainstormed.

Anyway, this is not an either/or situation: even though there are
currently compatibility issues, one can perfectly well develop/adapt
such tools in a way that makes them work fine with AppArmor enabled.

Let's enable AppArmor so we cover at least the server use case and the
low-hanging fruits of the desktop one, and figure out later where we
should put our efforts for securing the desktop, once the dust has
settled and next generation solutions have matured and been integrated
in Debian.

Will this prevent users from using another Linux Security Module?
-----------------------------------------------------------------

Some "minor" Linux Security Modules, such as Yama, live perfectly well
with others.

But currently it is not possible to enable several of the major
security modules. There's been (slow) work in progress to fix this for
a while, but it has picked up recently and there is a lot of interest
to have, say, AppArmor and SELinux stackable:

https://lwn.net/Articles/719731/

Now, every user will still be able to opt-out from AppArmor and
instead enable their preferred LSM.

What does upstream look like?
-----------------------------

The upstream project is almost 20 years old, very mature and
cooperative with Debian. E.g. the upstream release schedule has been
adjusted both for Jessie and Stretch to accommodate Debian's
schedule nicely.

Regarding who does the work:

- Canonical employees do most of the kernel work. They also maintain
the library and other C code, e.g. the policy parser.

- The Python utilities are primarily maintained by openSUSE's
Christian Boltz.

- Maintaining AppArmor policy is a cross-distro team effort, mostly
done by Debian, Ubuntu and openSUSE people.

How much will we depend on Canonical's business priorities?
-----------------------------------------------------------

Given Canonical employees do the greatest part of the work upstream:
indeed, we will. I see two main concerns about this:

Long-term reliability: this funding could run out some day.
I personally am not overly concerned, as Canonical has been investing
a lot into products (Snappy, LXC/LXD) that strongly depend on AppArmor
in the last few years.

Power imbalance: the company that does so much of the work has great
power over the priorities of the upstream project. This is the case
for a large amount of critical software we ship, so like it or not, it
is something we are living with already. AppArmor developers employed
by Canonical have shown great willingness in cooperating with Debian
in the last few years, so I'm confident that our contributions will be
welcome for the foreseeable future, whenever we need to adapt the
software to our needs. But of course management/business decisions can
change this at any time.

No thanks: I've tried AppArmor and it broke stuff too often
-----------------------------------------------------------

Sorry about that. I think this has had three main causes so far, that
all share one single root cause i.e. "AppArmor is not enabled by
default" (chicken'n'egg!):

1. Most package maintainers don't test packages with AppArmor before
uploading, so users notice breakage that could easily be avoided.

2. The huge majority of our users are not affected by breakage caused
by AppArmor, so we handle such breakage in a way that saves
maintainers' time: e.g. in many cases I've personally preferred to
wait for my fixes to AppArmor profiles to be approved and merged
upstream before applying them in Debian.

Once AppArmor is enabled by default, as far as I'm concerned
I don't plan to wait for upstream review before fixing regressions
in testing/sid.

3. The huge majority of our users are not affected by breakage caused
by AppArmor, so such breakage was kinda acceptable and thus we
*sometimes* preferred to give a specific AppArmor profile more
exposure to testers, even if it had a couple known issues, in order
to identify problems and help stabilize it (e.g. Tor, libvirt).

I think we will need to be more conservative once AppArmor is
enabled by default, i.e. profiles that break functionality too much
or too often should not be enabled by default.

Doesn't AppArmor need out-of-tree kernel patches?
-------------------------------------------------

Yes and no.

Historically, the mainline Linux kernel has supported a rather small
subset of the AppArmor mediation made possible by the out-of-tree
kernel patch. This made the value of enabling AppArmor smaller than it
could be (e.g. LXC is not well confined in Debian: #750106), and
smaller than it is in distros that apply the out-of-tree kernel patch
(such as Ubuntu).

Still, even with the set of features available in mainline Linux
*today*, IMO enabling AppArmor already has a good cost/benefit ratio
for Debian and our users. I'm not proposing we apply out-of-tree
AppArmor kernel patches.

Thankfully, the AppArmor kernel developers recently changed how they
proceed: new features are now added to Linux mainline before they
reach Ubuntu, so I'm confident that this situation will get better and
better in the future, and Buster's kernel will support tons of new
AppArmor mediation types compared to Stretch.

How can I help?
---------------

* Enable AppArmor on your Debian systems:
https://wiki.debian.org/AppArmor/HowToUse

* If you maintain a package for which we ship AppArmor policy in
Debian: test it with AppArmor enabled before uploading.

Ask for help if needed:
https://wiki.debian.org/AppArmor/Reportbug#Usertags

* Join the team:
https://wiki.debian.org/AppArmor/Contribute

* Talk to us:
<pkg-appa...@lists.alioth.debian.org>

Credits
-------

A huge thank you to the people who reviewed this text, provided
valuable input that I took into account & integrated, and helped me
change my mind here and there: Christian Boltz, gregoa, Guido Günther,
Jamie Strandboge, John Johansen, Sebastien Delafond, Simon McVittie
and Solveig! Sorry to those I forgot. I shamelessly stole bits of text
they wrote. I absolutely do *not* imply they endorse this proposal.

Thanks a lot to my pkg-apparmor team-mates, to Kees Cook who
introduced AppArmor in Debian in the first place, and to all AppArmor
contributors upstream and in other distros :)

Cheers,
--
intrigeri

[Niels Thykier]

intrigeri:

Hi,

Overall, this sounds like an interesting proposal and personally, I
agree that I think the Debian Linux ports would be better off with an
LSM enabled by default.

> What's the cost for Debian users?
> ---------------------------------
>
> AppArmor unavoidably breaks functionality from time to time: e.g.
> new versions of software we package (or of their dependencies)
> regularly start needing access to new file locations.

s/AppArmor/Any LSM/

Can we integrate these LSM policies into our testing frameworks (e.g.
autopkgtests), so we can start having automated tests of even basic
functionality. Or will that happen "out of the box" if we enable it by
default (and, possibly, enable it on our test hosts)?

Thanks,
~Niels

[Simon McVittie]

On Sat, 05 Aug 2017 at 06:50:00 +0000, Niels Thykier wrote:
> Can we integrate these LSM policies into our testing frameworks (e.g.
> autopkgtests), so we can start having automated tests of even basic
> functionality. Or will that happen "out of the box" if we enable it by
> default (and, possibly, enable it on our test hosts)?

In practice probably only if our test hosts change from being lxc
containers to being full virtual machines, which would be good for
other aspects of test coverage too (flatpak/bubblewrap currently skip
many of their autopkgtests, and so does debootstrap).

AppArmor has some amount of support for being used inside a privileged
lxc container, but it's very new and I'm not sure it's going to be a
particularly accurate representation of what would happen on real
hardware. (One of my colleagues looked at lxc + apparmor recently and
was able to make it work, but not trivially.)

S

[Antonio Terceiro]

On Sat, Aug 05, 2017 at 11:30:30AM +0100, Simon McVittie wrote:
> On Sat, 05 Aug 2017 at 06:50:00 +0000, Niels Thykier wrote:
> > Can we integrate these LSM policies into our testing frameworks (e.g.
> > autopkgtests), so we can start having automated tests of even basic
> > functionality. Or will that happen "out of the box" if we enable it by
> > default (and, possibly, enable it on our test hosts)?
>
> In practice probably only if our test hosts change from being lxc
> containers to being full virtual machines, which would be good for
> other aspects of test coverage too (flatpak/bubblewrap currently skip
> many of their autopkgtests, and so does debootstrap).

FWIW, my last upload of debci has a prototype qemu backend. I want to
switch to that sooner rather than later.

[Christoph Biedl]

intrigeri wrote...

> tl;dr: I hereby propose we enable AppArmor by default in testing/sid,
> and decide one year later if we want to keep it this way in the
> Buster release.

I really appreciate your approach of trying this out while being
prepared this might turn out to be a bad idea. Or: Promoting an idea
without being pushy about it.

So while adding another security layer is certainly something to
consider, I'm as well interested in whether this is feasible for a
generic-purpose distribution like Debian. The worst thing that could
happen was people will have to do the counterpart of chmod 777. Then it
was a bad idea, but we (as in Debian) have substantiation for such a
claim.

Christoph

[Holger Levsen]

On Fri, Aug 04, 2017 at 07:31:36PM -0400, intrigeri wrote:
> tl;dr: I hereby propose we enable AppArmor by default in testing/sid,
> and decide one year later if we want to keep it this way in the
> Buster release.

loving it, please go ahead! ;-)


--
cheers,
Holger

[Dr. Bas Wijnen]

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On Sat, Aug 05, 2017 at 06:28:20PM +0200, Christoph Biedl wrote:
> intrigeri wrote...
>
> > tl;dr: I hereby propose we enable AppArmor by default in testing/sid,
> > and decide one year later if we want to keep it this way in the
> > Buster release.
>

> [...] while adding another security layer is certainly something to

> consider, I'm as well interested in whether this is feasible for a
> generic-purpose distribution like Debian.

Enabling it by default doesn't mean it can't be switched off, right? I think
it makes a lot of sense to enable something like this by default, and in fact I
can't think of a situation where you would not want it, but indeed users should
be able to set their system up without it if they so wish.

> The worst thing that could happen was people will have to do the counterpart
> of chmod 777. Then it was a bad idea, but we (as in Debian) have
> substantiation for such a claim.

Yes, we should certainly avoid that; if it looks like that is happening, we
should abort the operation. But from the well written plan, it sounds to me
like this is unlikely to be the case.

So just to be clear: Yes, please enable AppArmor by default.

Thanks,
Bas
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[Andrey Rahmatullin]

On Sun, Aug 06, 2017 at 07:28:08AM +0000, Dr. Bas Wijnen wrote:
> I can't think of a situation where you would not want it

The "I don't want yet another thing that can cause subtle breakages and
doesn't give me anything" situation (see disabling selinux after install
on RH systems).

--
WBR, wRAR

[Moritz Mühlenhoff]

intrigeri <intr...@debian.org> schrieb:

> Still, even with the set of features available in mainline Linux
> *today*, IMO enabling AppArmor already has a good cost/benefit ratio
> for Debian and our users. I'm not proposing we apply out-of-tree
> AppArmor kernel patches.

I'd expect that a lot of the AppArmor profiles currently in use are
coming from Ubuntu or OpenSUSE. If one of those profiles relies
on features which are not upstreamed on the kernel end, how's
that handled?

Cheers,
Moritz

[intrigeri]

Dr. Bas Wijnen:

> Enabling it by default doesn't mean it can't be switched off, right?

Yes, passing apparmor=0 on the kernel command line will turn it off.

Cheers,
--
intrigeri

[intrigeri]

Hi,

Moritz Mühlenhoff:

> I'd expect that a lot of the AppArmor profiles currently in use are
> coming from Ubuntu or OpenSUSE.

Yes, historically (most of them are now maintained via a cross-distro
collaborative effort that a few Debian people participate in).

> If one of those profiles relies on features which are not upstreamed
> on the kernel end, how's that handled?

Rules that are not supported by the running kernel are silently
ignored, i.e. the operation is allowed.

For example, the profile for ping(8) specifies:

network inet raw,
network inet6 raw,

In Ubuntu, and in Debian once network socket mediation support lands
in Linux mainline, this means "no socket(2) based operation is allowed
except inet and inet6 raw sockets". In current Debian, no network
mediation by AppArmor is available so all socket(2) based operations
are allowed.

This way:

* the same profile can be shared between distros, regardless of
whether they apply not-upstreamed-yet AppArmor kernel patches;

* once new AppArmor features land in Linux mainline, we automatically
benefit from stronger confinement that's already implemented in our
AppArmor policy.

Cheers,
--
intrigeri

[Christian Seiler]

On 08/06/2017 05:32 PM, intrigeri wrote:
> Moritz Mühlenhoff:

>> If one of those profiles relies on features which are not upstreamed
>> on the kernel end, how's that handled?
>
> Rules that are not supported by the running kernel are silently
> ignored, i.e. the operation is allowed.

Is there at least a warning during the load of the profile? Consider a
local sysadmin that creates an own profile for a specific service they
run - and assume that AppArmor will confine it. But because the
kernel doesn't support a specific thing yet the confinement will be
incomplete. Which is probably better than not having AppArmor, and is
probably also OK for profiles shipped with the distribution and / or
upstream software - but not necessarily a good idea for something an
admin touches themselves.

Or, conversely, is there a possibility to add a flag to the AppArmor
profile to say "fail to load it if something is not understood"? In
that case all profiles shipped by Debian would not include that (for
interoperability reasons) but it could be documented that as a best
practice for admins they should use that flag so that they can be
sure that all protections they specified are actually affected.

Another thing to consider: if a profile is too restrictive, but the
part that is too restrictive isn't in the upstream kernel yet, then
things could break if you upgrade the kernel to a newer version from
e.g. backports later on. How would you deal with that kind of
breakage during the lifetime of a stable release?

Regards,
Christian

[Clint Byrum]

Excerpts from intrigeri's message of 2017-08-04 19:31:36 -0400:

> - Apparently Ubuntu users have been coping with AppArmor enforced
> by default for 9 years. I see no reason why Debian users would not.
>

This is really important. A few packages in Ubuntu largely differ from
Debian because they have an AppArmor profile that hasn't yet been
incorporated into Debian. That's a profile that is already tested and
used by many Ubuntu users, and should work well with a Debian system.

I found your proposal thorough and reasonable. I hope it happens.

[Moritz Mühlenhoff]

Christian Seiler <chri...@iwakd.de> schrieb:

> Another thing to consider: if a profile is too restrictive, but the
> part that is too restrictive isn't in the upstream kernel yet, then
> things could break if you upgrade the kernel to a newer version from
> e.g. backports later on. How would you deal with that kind of
> breakage during the lifetime of a stable release?

Agreed, that was pretty much my concern. Ideally the feature set
used would also be controlled by the apparmor userspace side.

Also, I'm wondering about the status of kernel support which is
currently not upstreamed: intrigeri mention that new features are
now added to Linux mainline. Was there ever an attempt to upstream
those existing patches (e.g. for network socket mediation); was it
NACKed by upstream for conceptual problems or was it simply never
attempted due to time/resource constraints?

Cheers,
Moritz

[Ritesh Raj Sarraf]

On Fri, 2017-08-04 at 19:31 -0400, intrigeri wrote:
> tl;dr: I hereby propose we enable AppArmor by default in testing/sid,
> and decide one year later if we want to keep it this way in the
> Buster release.

On most systems, people tend to disable LSM first. Because many a times
an inadequate policy hinders the use of the tool. And on the desktop
machine this becomes more common an issue.

On the SELinux side, there used to be a nice reporting tool for the
desktop, setroubleshoot. It used to alert (graphical and console) any
policy violations.

A DE agnostic alert tool would be necessary for wide adoption of any
LSM implementation.

In my opinion, we should start with an alert tool (to report policy
violations), and a handful of server packages.


--
Given the large number of mailing lists I follow, I request you to CC
me in replies for quicker response

[Jeremy Bicha]

On Mon, Aug 7, 2017 at 8:28 AM, Ritesh Raj Sarraf <r...@debian.org> wrote:
> On most systems, people tend to disable LSM first. Because many a times
> an inadequate policy hinders the use of the tool. And on the desktop
> machine this becomes more common an issue.

While there are plenty of people who recommend disabling SELinux by
default on Fedora/RHEL (at least in the past), it's far, far less
common to hear about anyone recommending disabling AppArmor. And
that's one reason AppArmor is being proposed here and not SELinux.

Thanks,
Jeremy Bicha

[Ritesh Raj Sarraf]

Hello Jeremy,

On Mon, 2017-08-07 at 08:47 -0400, Jeremy Bicha wrote:
> While there are plenty of people who recommend disabling SELinux by
> default on Fedora/RHEL (at least in the past), it's far, far less
> common to hear about anyone recommending disabling AppArmor. And
> that's one reason AppArmor is being proposed here and not SELinux.

They both serve the same purpose; to confine a running process, by the
mandated policy. The core problem is the policy, which may/will not fit
all users irrespective of the LSM implementation they use.

The previous adopters were all targeted distributions and it still took
some time for users to get accustomed to it.

RHEL confined most server packages. For the rest of the system, I
believe it ran unconfined. And they used to have a decent reporting
tool for policy violations.

SUSE used to have a robust LSM integration into YaST.

For a general purpose distribution like Debian, intri's proposal on the
server side is good. We could definitely leverage the policy files from
Ubuntu and SUSE.

But for desktop users, I worry this would cause more pain.
But I see there's an apparmor-notify package. Maybe that is the answer.
I'll check that out some time soon.


Thanks,
Ritesh

--
Ritesh Raj Sarraf | http://people.debian.org/~rrs
Debian - The Universal Operating System

[Jeremy Bicha]

On Mon, Aug 7, 2017 at 9:08 AM, Ritesh Raj Sarraf <r...@debian.org> wrote:
> But for desktop users, I worry this would cause more pain.

My point is that it hasn't so far and Ubuntu and SUSE has had millions
of people using it for a decade, more or less.

Thanks,
Jeremy Bicha

[Garrett R.]

I have Ubuntu running on another box. I haven't had any trouble (to my knowledge) that has been caused from apparmor. Ubuntu being perceived as an entry OS for linux, I would think Canonical wouldn't have included it if it would introduce pain to desktop users. What sort of "pain" might apparmor cause?

I assumed apparmor was also on debian. If it isn't, doesn't this make Ubuntu significantly more secure than a debian installation?

[Chris Lamb]

Hi intrigeri,

> tl;dr: I hereby propose we enable AppArmor by default in testing/sid,
> and decide one year later if we want to keep it this way in the
> Buster release.

Thanks for such a comprehensive and compelling write-up :)

> * Enable AppArmor on your Debian systems:
> https://wiki.debian.org/AppArmor/HowToUse

$ sudo aa-status | head -n2
apparmor module is loaded.
49 profiles are loaded.

(Well, I should take more risks, right…?)

> * If you maintain a package for which we ship AppArmor policy in
> Debian: test it with AppArmor enabled before uploading.

Related to this, most of my packages are 'server'-ish and it feels
like some of the hardening features are also/already covered by my
systemd .service files.

Should/could I be also reimplementing these in AppArmor for defense
in depth or any comments in this general area?


Best wishes,

--
,''`.
: :' : Chris Lamb
`. `'` la...@debian.org / chris-lamb.co.uk
`-

[intrigeri]

Hi,

Chris Lamb:

> Related to this, most of my packages are 'server'-ish and it feels
> like some of the hardening features are also/already covered by my
> systemd .service files.

Quite possibly :)

> Should/could I be also reimplementing these in AppArmor for defense
> in depth or any comments in this general area?

You surely could, but reimplementing exactly the same protections is
probably not the best use of your time.

Now, each of systemd and AppArmor can do hardening stuff that the
other doesn't support (at all or nicely), so sometimes it's good to do
a little bit of both.

What I would do these days:

1. Use the simplest of systemd's hardening features (e.g.
Protect{Home,System}=, Private{Devices,Tmp,Network}=,
CapabilityBoundingSet=) to their full extend.

Not many unit files we ship do that yet. Generally these
improvements can be implemented upstream and benefit users of
systemd on other distros :)

2. For finer-grained mediation of filesystem access, I will use
AppArmor that I find much nicer to implement, use and debug:

* To limit write access, with systemd one must set
ReadOnlyDirectories=/ and then a bunch of ReadWriteDirectories=
directives. It works fine once you're done, but I found it hard
to implement and debug this setup: with AppArmor I have clear
logs about what access the service was denied, and with systemd
I have no such thing. This probably explains why only one
systemd unit installed on my system bothers doing so.

* To limit read access, AFAIK with systemd one can only list
forbidden places and everything else is allowed by default.
No wonder InaccessiblePaths= is not used by any unit installed
on my system.

Cheers,
--
intrigeri

[Russ Allbery]

intrigeri <intr...@debian.org> writes:

> You surely could, but reimplementing exactly the same protections is
> probably not the best use of your time.

> Now, each of systemd and AppArmor can do hardening stuff that the
> other doesn't support (at all or nicely), so sometimes it's good to do
> a little bit of both.

> What I would do these days:

> 1. Use the simplest of systemd's hardening features (e.g.
> Protect{Home,System}=, Private{Devices,Tmp,Network}=,
> CapabilityBoundingSet=) to their full extend.

> Not many unit files we ship do that yet. Generally these
> improvements can be implemented upstream and benefit users of
> systemd on other distros :)

To this, I would add SystemCallFilters, which is immensely powerful and
much easier to use now than it was originally. I think a lot of the
benefit of hardening for a lot of network daemons would come from tested
use of SystemCallFilters (probably only using the @group syntax). This
blocks so many local privilege escalation bugs in random Linux syscalls,
making an RCE in a daemon running as a non-root user much less scary.

(For those not familiar, this is seccomp filtering, except about as easy
to use as seccomp can be. It's not as good as a carefully crafted full
seccomp policy, but you can get quite a bit of mileage out of some fairly
simple rules.)

Note that SystemCallFilters was very difficult to use in jessie since the
syscall groups weren't implemented yet. I would not recommend trying to
use it if targeting anything older than stretch.

> 2. For finer-grained mediation of filesystem access, I will use
> AppArmor that I find much nicer to implement, use and debug:

Agreed -- AppArmor seems to offer much more granular control over file
system access in a way that's quite a bit easier to configure.
ProtectSystem is great, but the bits that list specific files or paths are
somewhat harder to use and less useful.

--
Russ Allbery (r...@debian.org) <http://www.eyrie.org/~eagle/>

[intrigeri]

Hi,

Ritesh Raj Sarraf:

> But I see there's an apparmor-notify package.

Sadly it's not well integrated in Debian currently.

Root cause of the problem:
https://bugs.launchpad.net/apparmor/+bug/1597671

Short term workaround: https://bugs.debian.org/759604

> Maybe that is the answer.

I suspect the expected benefits are not worth the effort. I'd rather
see us put efforts in automated testing (to identify & fix bugs before
they affect less tech-savvy users) and in fixing actual UX
stumbling blocks.

(apparmor-notify is merely a glorified log reader displaying low-level
technical details in a GUI. So the info it gives people who are not
tech-savvy is essentially "the problem I'm experiencing might have
something to do with AppArmor". I suspect they will have a harder time
interacting with our BTS than copy'n'pasting the corresponding
AppArmor logs, and apparmor-notify won't change that.)

Cheers,
--
intrigeri

[intrigeri]

Christian Seiler:

> On 08/06/2017 05:32 PM, intrigeri wrote:
>> Rules that are not supported by the running kernel are silently
>> ignored, i.e. the operation is allowed.

> Is there at least a warning during the load of the profile?

There used to be a warning, but it was causing lots of confusion in
Debian: users were wondering if *any* of the AppArmor profile that
caused warning was applied at all. So in Jessie we decided to hide
these warnings.

> Or, conversely, is there a possibility to add a flag to the AppArmor
> profile to say "fail to load it if something is not understood"? In
> that case all profiles shipped by Debian would not include that (for
> interoperability reasons) but it could be documented that as a best
> practice for admins they should use that flag so that they can be
> sure that all protections they specified are actually affected.

If we're fine with relying purely on documentation to address this
problem for sysadmins writing their own profiles, then we can suggest
they use the existing apparmor_parser options about this:

alias apparmor_parser='apparmor_parser --warn=rules-not-enforced --warn=rule-downgraded'

… and then no new code needs to be written :)

Would that be good enough in your opinion?

Thanks for all this useful input!

Cheers,
--
intrigeri

[Christian Seiler]

On 08/09/2017 10:33 PM, intrigeri wrote:
> Christian Seiler:
>> On 08/06/2017 05:32 PM, intrigeri wrote:
>>> Rules that are not supported by the running kernel are silently
>>> ignored, i.e. the operation is allowed.
>
>> Is there at least a warning during the load of the profile?
>
> There used to be a warning, but it was causing lots of confusion in
> Debian: users were wondering if *any* of the AppArmor profile that
> caused warning was applied at all. So in Jessie we decided to hide
> these warnings.

Good to know. And since this kind of warning is more likely to lead
people to disable AppArmor altogether because they want to get rid
of the message and are scared by it, you're probably right in
removing that warning, because limited AppArmor protections are
probably still better as a defense-in-depth strategy than none at
all.

>> Or, conversely, is there a possibility to add a flag to the AppArmor
>> profile to say "fail to load it if something is not understood"? In
>> that case all profiles shipped by Debian would not include that (for
>> interoperability reasons) but it could be documented that as a best
>> practice for admins they should use that flag so that they can be
>> sure that all protections they specified are actually affected.
>
> If we're fine with relying purely on documentation to address this
> problem for sysadmins writing their own profiles, then we can suggest
> they use the existing apparmor_parser options about this:
>
> alias apparmor_parser='apparmor_parser --warn=rules-not-enforced --warn=rule-downgraded'
>
> … and then no new code needs to be written :)
>
> Would that be good enough in your opinion?

If that documentation is easy enough to find: sure, yes.

Speaking of: are there any good introductions for AppArmor? Not
necessarily for me personally (I've had some experience with
SELinux, so I recon I'll figure AppArmor out easily enough), but
for beginners? Something I can point friends of mine to? Ideally
something that doesn't just describe the syntax, but also best
practices? When I first ran into SELinux I found a couple of
tutorials which did things where I thought, well this doesn't
seem quite well enough thought out, I can think of a couple of
corner cases where this would fainot do what it was advertising,
only to later find that my impression was true and people with
even more knowledge of SELinux talked about not doing the things
I had seen in these tutorials, and suggesting more sensible
things as best practices. Unfortunately the writings by the real
experts were not in tutorial form. For buster, if AppArmor is
enabled by default (which btw. I'm in favor of, in case that was
not clear), I think we should take care to nudge people towards
the resources that describe best practices.

Regards,
Christian

[intrigeri]

Hi,

[John, there's a question for you at the bottom, but you probably have
useful input about the first part of the discussion below too.]

Moritz Mühlenhoff:

> Christian Seiler <chri...@iwakd.de> schrieb:
>> Another thing to consider: if a profile is too restrictive, but the
>> part that is too restrictive isn't in the upstream kernel yet, then
>> things could break if you upgrade the kernel to a newer version from
>> e.g. backports later on. How would you deal with that kind of
>> breakage during the lifetime of a stable release?

> Agreed, that was pretty much my concern.

Thank you so much for highlighting problems I had missed! :)

A simple, but not entirely satisfying answer is:

1. Gather info about how real this problem has been in practice for
Ubuntu: they frequently update their kernel for various already
released distros with the latest AppArmor bits. I think they
occasionally have to update other packages accordingly to adjust
AppArmor policy. I don't know how often this happens. I'll ask them
to compile a list of such stable updates.

2. Evaluate for a year how it goes for Stretch + Linux from backports.

Desktop: I'm in a good place to provide data points, as Tails
generally ships this combination and we exercise the vast majority
of the desktop AppArmor stuff that's in Debian.

Server: sorry, I use the stable kernel except on bare-metal
virtualization hosts. But I think (1) will give us enough data on
this front.

3. Depending on what (1) and (2) tell us, decide whether "we might
have to update AppArmor policy from time to time in stable
point-releases or backports" is good enough… keeping in mind that
other distros are already dealing with the exact same problem, so
we won't have to do this alone. And if it's not good enough:

> Ideally the feature set used would also be controlled by the
> apparmor userspace side.

If we need to go this far: apparmor_parser has a --features-file
option that we could leverage to tie the feature set being used to
something else than the version of the running kernel, e.g.
with a file shipped in a new package built from src:linux with
appropriate versioned dependencies.

> Also, I'm wondering about the status of kernel support which is
> currently not upstreamed: intrigeri mention that new features are
> now added to Linux mainline. Was there ever an attempt to upstream
> those existing patches (e.g. for network socket mediation); was it
> NACKed by upstream for conceptual problems or was it simply never
> attempted due to time/resource constraints?

I'm not sure, so I'll let the main AppArmor kernel developer (John,
Cc'ed) answer this.

Cheers,
--
intrigeri

[John Johansen]

There are several distinct issues when dealing with stable release
support. There are basically 3 different potentially moving components
to consider

kernel - changes may result in a change in the supported feature
set.

policy - a package may backport/drop in policy that was developed on
a different feature set.

user space - changes/updates may be required to support new features
in a kernel or policy.

The question specifically asks about, an updated kernel with a policy
that was developed under a different feature set, suddenly breaking
when a new kernel is run on an older system.

The kernel portion is designed to support multiple versions of
userspace policy abis, and while we have dropped some of the older
abis, it is done slowly. The current kernel code supports the abi from
2009. So newer kernels will support older policy and userspaces.

Similarly the userspace is designed to support multiple kernel
versions and abis, so a newer userspace (if it is SRUed for some
reason) can support older kernels.

This leaves us dealing with policy. As long as the policy has not been
changed, it is possible to force userspace to build policy for a
certain kernel feature set by specifying the feature file.

This can be done in the apparmor/subdomain.conf file

with this specified, policy should remain to be compiled as for the
older kernel, and the new kernel should support and enforce it under
that abi.

There is however a caveat that sometimes the kernel changes abi
behavior in ways that apparmor can not adapt to without policy
modification.

An example of this would be
commit 9f834ec18defc369d73ccf9e87a2790bfa05bf46
binfmt_elf: switch to new creds when switching to new mm

which resulted in a change of which cred the PROT_EXEC request was
made against. With the way the lsm hook is designed only the single
cred is available so it is not even possible for apparmor to emulate
the old behavior.

In this case older policy needed to be patched to support the newer
behavior if it was to be used on a newer kernel, and a flag was
added to the apparmor kernel features interface making it possible
for userspace to detect that the kernel contained the change.

Thankfully this type of change is has not been very common.

>> Ideally the feature set used would also be controlled by the
>> apparmor userspace side.
>
> If we need to go this far: apparmor_parser has a --features-file
> option that we could leverage to tie the feature set being used to
> something else than the version of the running kernel, e.g.
> with a file shipped in a new package built from src:linux with
> appropriate versioned dependencies.
>

the feature file can indeed be specified on the command line using
--feature-file, but from a support pov I think specifying it in the
config file

apparmor/subdomain.conf

would be better as then you don't have to mess with initscripts, unit
files, etc.

>> Also, I'm wondering about the status of kernel support which is
>> currently not upstreamed: intrigeri mention that new features are
>> now added to Linux mainline. Was there ever an attempt to upstream
>> those existing patches (e.g. for network socket mediation); was it
>> NACKed by upstream for conceptual problems or was it simply never
>> attempted due to time/resource constraints?
>
> I'm not sure, so I'll let the main AppArmor kernel developer (John,
> Cc'ed) answer this.
>

So current status of kernel support that hasn't been upstreamed is

4.13 - has most everything. It has the virtualized apparmorfs and
namespacing interfaces, the core rework, stacking, etc. It is
missing some key features, specifically signals, mount and
network mediation

4.14 - isn't fully decided yet, but it should pickup everything except
maybe the extended unix socket mediation

As for why upstreaming has taken so long, there have been several
reasons.

Time constraints have been the major issue. Certain new feature
development was certainly prioritized over getting existing out of
tree features upstream. There is recognition that this was the wrong
approach and there is now an upstream first policy.

In addition some of the delay has also been just a matter of letting
code bake and shaking out bugs. Some of the code has had several
revisions before we were happy with it and thought it ready to be
upstreamed.

Eg. the extension to securityfs to allow apparmor to virtualize its
policy dir has seen several revisions. And even changed approach
before the final version which was upstreamed in 4.13. It just took
some time for the code to bake and for the correct approach to settle
out. Ubuntu was willing to carry the dev code to get certain features
early, at an increased cost in support.

The dbus code went through several revisions as well. While the dbus
code doesn't require a lot from the kernel, it did have some influence
on the kernel support interfaces.

Another reason many of these features were delayed in their
upstreaming, is the apparmor core was being rewritten during their
development, which unfortunately resulted in there not being a clean
separation between the different parts of development. So while some
features could have been upstreamed sooner they had to wait for the
core changes to bake, or be rewritten to work with the existing
upstream code (time and resources weren't available for the rewrite).

None of the current development has been NAKed upstream. At times in
the past certain approaches have been NAKed, and required reworking.
This was largely around how apparmor interacted with the system and
modifications to the LSM hooks. That is not to say that some of the
existing out of tree features might not get naked or need some
reworking before they land upstream.

We have been trying to drive the diff down to 0, and we are now close.

[Raphael Hertzog]

Hi,

On Wed, 09 Aug 2017, Christian Seiler wrote:
> Speaking of: are there any good introductions for AppArmor? Not
> necessarily for me personally (I've had some experience with
> SELinux, so I recon I'll figure AppArmor out easily enough), but
> for beginners? Something I can point friends of mine to? Ideally
> something that doesn't just describe the syntax, but also best
> practices? When I first ran into SELinux I found a couple of

https://debian-handbook.info/browse/stable/sect.apparmor.html

Feedback welcome as bug reports against the debian-handbook package.
Note that this (still) describes the status of the package
in jessie, I have not yet finished to update the book for stretch.

Cheers,
--
Raphaël Hertzog ◈ Debian Developer

Support Debian LTS: https://www.freexian.com/services/debian-lts.html
Learn to master Debian: https://debian-handbook.info/get/

[Simon McVittie]

On Wed, 09 Aug 2017 at 17:17:17 -0700, John Johansen wrote:
> The dbus code went through several revisions as well. While the dbus
> code doesn't require a lot from the kernel, it did have some influence
> on the kernel support interfaces.

I'm curious: is the kernel side of D-Bus mediation specific to D-Bus,
or is it general-purpose support for mediating user-space activities
that look roughly the same shape as D-Bus?

S

[John Johansen]

The kernel side of D-Bus is generic except one flag.

It provides

- label/context info to the proc/<pid>/attr/current and SO_PEER_SEC
which are wrapped by the libapparmor api fns

- the query interface which allows user space to query policy that
is loaded into the kernel. The dbus code was the first consumer
so it helped shape what the interface looks like and how the
queries are constructed.

- a flag in the features advertising that dbus mediation support
is available. This last one currently is set by the kernel
but ideally would be enabled by the dbus code advising the
kernel module it is mediating.

For dbus mediation we assigned dbus its own class with in the
policydb. Queries to the dbus class need have the dbus query
structure, queries to other classes must follow those classes.
There is certainly some similarity between the classes, but
some of them are quite different from dbus.

[Simon McVittie]

On Thu, 10 Aug 2017 at 12:00:15 -0700, John Johansen wrote:
> but ideally would be enabled by the dbus code advising the
> kernel module it is mediating

"The" dbus code? There can be several parallel instances of dbus-daemon,
possibly different versions of the executable, certainly differently
configured, which can result in any combination of them having
AppArmor mediation enabled or disabled. For example a typical GNOME
laptop will have a system bus, a session bus for the system user
that runs the gdm greeter, and a session bus for the logged-in user
account.

It is meaningful to ask whether a specific dbus-daemon instance is
applying AppArmor mediation, and the latest development branches
advertise this by putting "apparmor" in the bus driver's Features
property. In general it isn't necessarily meaningful to say
"the dbus-daemons running on this kernel are applying AppArmor
mediation" because some of them might be an executable that doesn't
support it, and some of them might support it but have it disabled
in configuration.

So I think this is something that should be queried by asking each
dbus-daemon whether it is mediating, rather than by asking the kernel.

S

[John Johansen]

yep having a way to detect/ask individual deamons is the way to go.

I was merely commenting on that the current kernel flag not being
reflective of actual mediation. Which the dbus daemon is providing, and
it (they) should be what is setting the support status, whether in
kernel or by a different means.

Regardless we will be keeping the kernel flag for several years to
provide backwards compat for newer kernels on earlier releases.

[Wouter Verhelst]

It actually does give you anything, and debugging SELinux breakages is
fairly simple, with the fix usually being rather small.

For a while after RedHat started enabling SELinux by default on their
systems, a number of security researchers yelled "I've found an
exploitable bug in RedHat" for which the first step to exploit was
always "first, disable SELinux". I'm not saying that that's not a
problem, but it *does* show that using SELinux or AppArmor has benefits.

I think enabling an LSM by default is a good idea, and we should do it
if we can. The "subtle breakages" you mention are annoying for you, but
they can be a showstopper for an attacker, and that's a *good* thing.
Yes, obviously when we enable an LSM we should also make it easy for
users to understand that something is blocked by the LSM and explain to
them how they can unblock it if they want to. But just saying "it causes
issues, let's not" is the same as saying "permissions on the file system
causes issues, let's install to FAT32 everywhere", and that's just not a
good idea.

--
Could you people please use IRC like normal people?!?

-- Amaya Rodrigo Sastre, trying to quiet down the buzz in the DebConf 2008
Hacklab

[Lisandro Damián Nicanor Pérez Meyer]

On viernes, 4 de agosto de 2017 19:31:36 -03 intrigeri wrote:
> Hi!

>
> tl;dr: I hereby propose we enable AppArmor by default in testing/sid,
> and decide one year later if we want to keep it this way in the
> Buster release.
>

> My goals when initiating this discussion are:
>
> - Get a rough idea of what amount of effort the Debian project is
> happy (and able) to invest into such proactive security measures.
>
> - Learn about any relevant social & technical concerns I am not
> aware of.
>
> I don't expect we'll make a decision based on this very proposal:
> I expect the proposal will need to be refined, or abandoned, to take
> into account what we will learn during this preliminary discussion.

From the way you redacted this you made me want to try apparmor. That's
already a big compliment :-)

For what I read in the proposal and in the thread I do believe we need to at
least try this for a year.

So my +1 for going ahead.

--
Hi! I'm a .signature virus! Copy me into your ~/.signature, please!

Lisandro Damián Nicanor Pérez Meyer
http://perezmeyer.com.ar/
http://perezmeyer.blogspot.com/

[Chris Lamb]

Hi intri,

> tl;dr: I hereby propose we enable AppArmor by default in testing/sid,
> and decide one year later if we want to keep it this way in the
> Buster release.

So… in the spirit of taking (reversible!) risks, can you briefly outline
what's blocking us enabling this today? :)

[Chris Lamb]

Hey intri,

> 1. Use the simplest of systemd's hardening features (e.g.
> Protect{Home,System}=, Private{Devices,Tmp,Network}=,
> CapabilityBoundingSet=) to their full extend.
>
> Not many unit files we ship do that yet. Generally these
> improvements can be implemented upstream and benefit users of
> systemd on other distros :)

Indeed! :) For example, here I'm merging upstream's rather more locked
down unit file into the Debian one:

https://bugs.debian.org/871610


Regards,

[intrigeri]

Raphael Hertzog:
> https://debian-handbook.info/browse/stable/sect.apparmor.html

Thanks, added to https://wiki.debian.org/AppArmor#External_links :)

Cheers,
--
intrigeri

[intrigeri]

Hi,

Christian Seiler:

> On 08/09/2017 10:33 PM, intrigeri wrote:
>>> Or, conversely, is there a possibility to add a flag to the AppArmor
>>> profile to say "fail to load it if something is not understood"? In
>>> that case all profiles shipped by Debian would not include that (for
>>> interoperability reasons) but it could be documented that as a best
>>> practice for admins they should use that flag so that they can be
>>> sure that all protections they specified are actually affected.
>>
>> If we're fine with relying purely on documentation to address this
>> problem for sysadmins writing their own profiles, then we can suggest
>> they use the existing apparmor_parser options about this:
>>
>> alias apparmor_parser='apparmor_parser --warn=rules-not-enforced --warn=rule-downgraded'
>>
>> … and then no new code needs to be written :)
>>
>> Would that be good enough in your opinion?

> If that documentation is easy enough to find: sure, yes.

For now, I've added this to
https://wiki.debian.org/AppArmor/Contribute#Create_new_profiles

Once we've made progress on the documentation front (see below)
we can ensure that whatever resource we recommend documents this.

> Speaking of: are there any good introductions for AppArmor?

> […]

> For buster, if AppArmor is enabled by default (which btw. I'm in
> favor of, in case that was not clear), I think we should take care
> to nudge people towards the resources that describe best practices.

There are a number of AppArmor tutorials aimed at beginners. We link
to a few of them on https://wiki.debian.org/AppArmor#External_links.
but the mere fact that there are 7 of them, with greatly overlapping
content and no indication of who the target audience is, suggests we
don't have a good answer to your question yet.

So I've filed #874873 (severity: important) to keep this on our radar.

Thanks for caring,
cheers!
--
intrigeri

[intrigeri]

Hi John et al,

John Johansen:

> On 08/09/2017 02:31 PM, intrigeri wrote:
>> Moritz Mühlenhoff:
>>> Christian Seiler <chri...@iwakd.de> schrieb:
>>>> Another thing to consider: if a profile is too restrictive, but the
>>>> part that is too restrictive isn't in the upstream kernel yet, then
>>>> things could break if you upgrade the kernel to a newer version from
>>>> e.g. backports later on. How would you deal with that kind of
>>>> breakage during the lifetime of a stable release?
>>
>>> Agreed, that was pretty much my concern.
>>
>> Thank you so much for highlighting problems I had missed! :)
>>
>> A simple, but not entirely satisfying answer is:
>>
>> 1. Gather info about how real this problem has been in practice for
>> Ubuntu: they frequently update their kernel for various already
>> released distros with the latest AppArmor bits. I think they
>> occasionally have to update other packages accordingly to adjust
>> AppArmor policy. I don't know how often this happens. I'll ask them
>> to compile a list of such stable updates.

> [...]

> The question specifically asks about, an updated kernel with a policy
> that was developed under a different feature set, suddenly breaking
> when a new kernel is run on an older system.

Right.

Below you elaborate about ABI compatibility between the kernel,
userspace and policy. Thanks, I've learned a lot!

But even more specifically, the question was about policy updates
mandated to avoid breaking *confined applications* when upgrading to
a kernel that mediates more interfaces than the one in Debian stable.

Christian Seiler put it clearly (quoted above) but here's a more
practical example: say 1. D-Bus mediation lands in Linux
4.15 (totally made up, but this would be nice!); 2. I run Debian
Stretch; 3. I have to run Linux 4.15+ from stretch-backports (e.g.
on a shiny laptop that needs recent drivers). Then any AppArmor
profile shipped in Debian Stretch that was developed without D-Bus
mediation in mind can potentially start breaking the application
it confines.

So our questions to Ubuntu & other distros are:
How have you been dealing with such problems in the past few years?
How often did you have to update packages in a stable release in
order to fix them?

Now, simply enabling AppArmor by default during the Buster development
cycle will give us some of the answers: given many AppArmor features
will land in Linux in the next months/years, we *will* notice if our
policy is outdated :)

>>> Ideally the feature set used would also be controlled by the
>>> apparmor userspace side.
>>
>> If we need to go this far: apparmor_parser has a --features-file
>> option that we could leverage to tie the feature set being used to
>> something else than the version of the running kernel, e.g.
>> with a file shipped in a new package built from src:linux with
>> appropriate versioned dependencies.

> the feature file can indeed be specified on the command line using
> --feature-file, but from a support pov I think specifying it in the
> config file

> apparmor/subdomain.conf

Do you mean /etc/apparmor/parser.conf?
I can't find anything related in subdomain.conf(5).

> would be better as then you don't have to mess with initscripts, unit
> files, etc.

Absolutely. I guess we would want a package built from src:apparmor to
ship that conffile containing "features-file XYZ", where XYZ encodes
the feature set supported by the policy in the version of Debian this
src:apparmor was built for. Which raises a number of technical and
policy questions, not all of them trivial, so I want to first check
whether we really need to go that far (see above).

> 4.14 - isn't fully decided yet, but it should pickup everything except
> maybe the extended unix socket mediation

Just curious: does this "everything except" include D-Bus mediation?

> There is recognition that this was the wrong approach and there is
> now an upstream first policy.

This, along with the vivid collaboration I see between the GNOME and
Ubuntu projects these days, is very good news :)

Cheers,
--
intrigeri

[John Johansen]

haha, I had a broader answer dealing with some of this and upon review
had decided the question was about a newer kernel on an older release,
and it would be best to have concise answers around that :)

> Christian Seiler put it clearly (quoted above) but here's a more
> practical example: say 1. D-Bus mediation lands in Linux
> 4.15 (totally made up, but this would be nice!); 2. I run Debian
> Stretch; 3. I have to run Linux 4.15+ from stretch-backports (e.g.
> on a shiny laptop that needs recent drivers). Then any AppArmor
> profile shipped in Debian Stretch that was developed without D-Bus
> mediation in mind can potentially start breaking the application
> it confines.
>

This is true, hence the suggestion to pin the feature set by
setting the features file in parser.conf This would prevent policy
from enforcing the dbus feature, and prevent the application
from breaking.

I will admit this is not ideal because it applies to all policy loaded
in the namespace (a container could have its own parser and flags)
unless policy is manually loaded with a flag to override it. Which
prevents policy that has been developed with the new feature from
taking advantage of it in this scenario.

There is some work to expose the feature set to policy which will let
policy conditionally choose which features it supports but I can't
promise when that work will land.

> So our questions to Ubuntu & other distros are:
> How have you been dealing with such problems in the past few years?
> How often did you have to update packages in a stable release in
> order to fix them?
>
> Now, simply enabling AppArmor by default during the Buster development
> cycle will give us some of the answers: given many AppArmor features
> will land in Linux in the next months/years, we *will* notice if our
> policy is outdated :)
>

So there are four separate components (kernel, userspace, policy,
application) to discuss here and different potential problems,
depending on the combination.

- kernel: If the kernel is backported and the feature set is pinned
there is a low likely hood of problems. As I addressed previously
there is the potential for a kernel to make changes beyond
apparmor's control that change how/what permission requests reach
apparmor and this can cause problems. Thankfully in practice this
has not happened often.

- apparmor userspace: Baring bugs, new userspaces should just work
with old kernels. Even if the feature set is not pinned, the
userspace will use the old kernel's feature set, so it is equivalent
to pinning.

- applications: When newer versions of applications are backported
they can break under old policy because they provide new features
that old policy wasn't designed for. Policy must be tested and
updated as part of an application backport/SRU.

- policy: The backporting of policy is the most problematic. New
policy shouldn't be dropped onto older applications without testing.

Also new policy may make use of features that are not supported by
an older userspace. In this case policy should be adjusted or a
newer apparmor userspace can be used. If the feature set is pinned
the apparmor userspace can gracefully downgrade unsupported features
so that new policy can work on older feature sets (you can also
configure it to warn or abort).

The single biggest problem is applications that would like to share
a single policy across multiple releases. Ie. they only want to
maintain a single policy for Stretch, and Buster but Stretch
apparmor userspace doesn't support some feature required in Buster.

Currently this requires backporting a newer apparmor userspace and
pinning of the feature set if the kernel changes.

There is work in progress to allow older parsers to recognize and
downgrade new features without directly supporting them but I can
not say when this will land.


Speaking from experience with Ubuntu, the kernel backports are seldom
problematic. We have seen the most issues around application
backports; either policy needing to be updated if it is not shipped as
part of the application, or the application policy requiring features
not supported by the apparmor userspace. This can be dealt with by
either editing the policy or backporting a newer apparmor userspace.

These issues should be and generally are caught during the SRU
process.

>>>> Ideally the feature set used would also be controlled by the
>>>> apparmor userspace side.
>>>
>>> If we need to go this far: apparmor_parser has a --features-file
>>> option that we could leverage to tie the feature set being used to
>>> something else than the version of the running kernel, e.g.
>>> with a file shipped in a new package built from src:linux with
>>> appropriate versioned dependencies.
>
>> the feature file can indeed be specified on the command line using
>> --feature-file, but from a support pov I think specifying it in the
>> config file
>
>> apparmor/subdomain.conf
>
> Do you mean /etc/apparmor/parser.conf?
> I can't find anything related in subdomain.conf(5).
>

ah yep, oops there used to be a subdomain.conf, back before Novell
renamed the project to AppArmor.

>> would be better as then you don't have to mess with initscripts, unit
>> files, etc.
>
> Absolutely. I guess we would want a package built from src:apparmor to
> ship that conffile containing "features-file XYZ", where XYZ encodes
> the feature set supported by the policy in the version of Debian this
> src:apparmor was built for. Which raises a number of technical and
> policy questions, not all of them trivial, so I want to first check
> whether we really need to go that far (see above).
>
>> 4.14 - isn't fully decided yet, but it should pickup everything except
>> maybe the extended unix socket mediation
>
> Just curious: does this "everything except" include D-Bus mediation?
>

No D-Bus mediation depends on the extended unix domain socket
mediation.

And I can now say that the extended unix socket mediation didn't make
it into 4.14 but everything else did. So you can use basic af socket
rules.

[Guido Günther]

Hi John,

very interesting read!

This (and somewhat related the next point) is the reason why policy
should be shipped by the application (that is the Debian package
containing the application), not in an apparmor-profiles package. This
makes sure the profile matches the application. Everything else calls
out for trouble.

Cheers,
-- Guido

[intrigeri]

Hi,

Chris Lamb:

> So… in the spirit of taking (reversible!) risks, can you briefly outline
> what's blocking us enabling this today? :)

Thanks for asking!

I've scheduled time on October 23-27 to:

1. identify what still prevents us from starting the proposed
experiment

2. fix all the problems identified in #1

3. document on the BTS what must be fixed between the time we start
the experiment and the time we decide what to do for the Buster
release

After this we should be in a very good position to go ahead and press
the big red button :)

The feedback I've received off-list and on the BTS so far has been
extremely encouraging: a few DDs enabled AppArmor on their systems,
reported bugs that were promptly fixed, and some of them successfully
fixed the bug themselves locally despite having no previous experience
with AppArmor… which is exactly how it should be for the LSM enabled
by default in Debian IMO. In at least one case we realized only after
I had fixed the bug and submitted a fix upstream that their own, local
workaround was identical to my own fix, which I find enlightening wrt.
the AppArmor learning curve.

Cheers,
--
intrigeri